The electrophoretic technique for the determination of lactate dehydrogenase (LD) isoenzymes has been refined and compared with other electrophoretic methods. New information has been reported which aids the interpretation of test results. Application of the same LD isoenzyme test to the analysis of serum samples from patients with testicular tumors provided useful information for the detection of the occurrence of these tumors and their metastasis. The same test was employed for the determination of LD isoenzyme pattern characteristics of various types of normal and of leukemic cells; to serve as markers for the early detection and follow-up or treatment of leukemia patients.